1. Field of the Invention
The present invention relates to master alloys for the modification of the micro-structure of aluminium-silicon casting alloys. Particularly, the present invention is related to a master alloy containing strontium, magnesium and aluminum for modifying the aluminum-silicon eutectic phase of hypoeutectic, eutectic and hypereutectic aluminum-silicon based casting alloys.
2. Discussion of the Background and Description of Related Art
The addition of strontium to other metals and alloys in order to improve the properties of the resultant alloy is known. Strontium is generally added to alloys either as a pure metal or in the form of a master alloy. The use of pure strontium has certain limitations. The metal readily oxidizes in a humid atmosphere and the presence of an oxide layer can inhibit the rate of dissolution of the strontium into the melt. Although the pure metal dissolves well in an aluminum-silicon-magnesium casting alloy melt between 675.degree.-725.degree. C., its dissolution rate decreases significantly at higher temperatures (725.degree.-775.degree. C.).
In U.S. Pat. No. 3,926,690, Morris et al. disclose that the addition of 0.01-0.5% strontium or calcium to an alloy of aluminum-magnesium-silicon provides an alloy with improved extrusion properties. In U.S. Pat. No. 4,394,348, Hardy et al. disclose the use of a master alloy containing strontium peroxide to introduce strontium into an aluminum bearing alloy to provide a finer grain alloy. Strontium is also known to be a superior modifier of the aluminum-silicon eutectic component of eutectic, hypereutectic and hypoeutectic aluminum-silicon casting alloys.
In U.S. Pat. No. 4,108,646, Gennone et al. disclose a powder or compact containing strontium-silicon and an aluminous material for use as a master composition. Strontium-containing master alloys are also disclosed in U.S. Pat. No. 4,009,026 and 4,185,999. British Pat. No. 1,520,673 discloses a master alloy of aluminum-silicon-strontium.
Known strontium master alloys, with increased amounts of strontium, have the disadvantage of low dissolution rates into aluminum-silicon casting alloys. Although master alloys with a lower strontium levels, such as aluminum-3.5% strontium, have rapid dissolution rates into aluminum-silicon casting alloys, larger quantities of the strontium containing master alloy must be added to achieve the desired strontium level in the melt.